The tightness of the fit between a shaft of an electronically-controlled turbocharger (ECT) and a rotor of the electric motor has competing demands. The fit should be tight enough that there is no relative motion between the two at the most demanding operating conditions anticipated. That is, operating conditions in which the torques applied to the shaft and rotor act to present the highest difference in torque urging them to rotate separately and elevated temperature conditions that can affect the fit by unequal thermal expansion. The fit must also resist separation at high rotational speeds due to centrifugal forces acting on the masses causing expansion as well as maintain connection over the entire operating range including extreme cold and hot conditions which can cause unequal thermal expansion. The fit should not be so tight such that stresses set up in the mated parts develop cracks. Furthermore, the fit should be as easy to assemble as possible. In some cases, it may be found useful to heat the rotor or to cool the shaft to facilitate assembly of an interference fit. If such thermal preparation can be avoided or the error rate of the assembly can be reduced, assembly cost is reduced. In prior ECTs, the rotor is fit over the shaft with a slip fit or an interference fit along the majority of the length over which the two are mated. In both the slip and interference fits the inside diameter of the rotor substantially equals the outside diameter of the shaft. With the former, there is a slight clearance and in the latter, there is a slight overlap meaning that the shaft diameter exceeds the rotor inside diameter so that when mated there is an interference. One of the problems of such a large surface area of engagement is that if there is any imperfection in the surface finish or concentricity, the two may become stuck at an intermediate position, i.e., not fully assembled. In addition, due to the minute tolerance ranges used for turbomachinery, cylindricity and roundness further complicate the fit. In the event of deviation in any of the above-described factors, the shaft and the rotor are non compliant.